Innodb存储引擎中的后台线程详解

1.maste thread

负责将缓冲池中的数据异步刷新到磁盘，保证数据的一致性。

2.io thread

负责io请求的回调处理。

1.0版本之前有4个io thread，负责write、read、insert buffer和log io thread

1.0.x开始，read thread和write thread分别增加到4个，不再使用innodb_file_io_threads参数，而是使用innodb_read_io_threads和innodb_write_io_threads

mysql> show variables like 'innodb_version'\g
*************************** 1. row ***************************
variable_name: innodb_version
        value: 5.6.25
1 row in set (0.00 sec)
 
mysql> show variables like 'innodb_%io_threads'\g
*************************** 1. row ***************************
variable_name: innodb_read_io_threads
        value: 4
*************************** 2. row ***************************
variable_name: innodb_write_io_threads
        value: 4
2 rows in set (0.00 sec)
 
mysql> show engine innodb status\g
*************************** 1. row ***************************
  type: innodb
  name:
status:
=====================================
2015-12-20 21:53:50 7fcaccfe8700 innodb monitor output
=====================================
per second averages calculated from the last 11 seconds
-----------------
background thread
-----------------
srv_master_thread loops: 93 srv_active, 0 srv_shutdown, 26243 srv_idle
srv_master_thread log flush and writes: 26336
----------
semaphores
----------
os wait array info: reservation count 393
os wait array info: signal count 347
mutex spin waits 297, rounds 8910, os waits 281
rw-shared spins 53, rounds 1590, os waits 50
rw-excl spins 13, rounds 1860, os waits 57
spin rounds per wait: 30.00 mutex, 30.00 rw-shared, 143.08 rw-excl
------------
transactions
------------
trx id counter 2545
purge done for trx's n:o < 2540 undo n:o < 0 state: running but idle
history list length 22
list of transactions for each session:
---transaction 0, not started
mysql thread id 5, os thread handle 0x7fcaccfe8700, query id 491 localhost root init
show engine innodb status
--------
file i/o
--------
i/o thread 0 state: waiting for i/o request (insert buffer thread)
i/o thread 1 state: waiting for i/o request (log thread)
i/o thread 2 state: waiting for i/o request (read thread)
i/o thread 3 state: waiting for i/o request (read thread)
i/o thread 4 state: waiting for i/o request (read thread)
i/o thread 5 state: waiting for i/o request (read thread)
i/o thread 6 state: waiting for i/o request (write thread)
i/o thread 7 state: waiting for i/o request (write thread)
i/o thread 8 state: waiting for i/o request (write thread)
i/o thread 9 state: waiting for i/o request (write thread)
pending normal aio reads: 0 [0, 0, 0, 0] , aio writes: 0 [0, 0, 0, 0] ,
 ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0
pending flushes (fsync) log: 0; buffer pool: 0
915 os file reads, 1496 os file writes, 1089 os fsyncs
0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s
-------------------------------------
insert buffer and adaptive hash index
-------------------------------------
ibuf: size 1, free list len 0, seg size 2, 0 merges
merged operations:
 insert 0, delete mark 0, delete 0
discarded operations:
 insert 0, delete mark 0, delete 0
hash table size 276671, node heap has 2 buffer(s)
0.00 hash searches/s, 0.00 non-hash searches/s
---
log
---
log sequence number 369664448
log flushed up to   369664448
pages flushed up to 369664448
last checkpoint at  369664448
0 pending log writes, 0 pending chkp writes
356 log i/o's done, 0.00 log i/o's/second
----------------------
buffer pool and memory
----------------------
total memory allocated 137363456; in additional pool allocated 0
dictionary memory allocated 96365
buffer pool size   8191
free buffers       53
database pages     8136
old database pages 2983
modified db pages  0
pending reads 0
pending writes: lru 0, flush list 0, single page 0
pages made young 9747, not young 4864716
0.00 youngs/s, 0.00 non-youngs/s
pages read 4212, created 12449, written 13322
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
no buffer pool page gets since the last printout
pages read ahead 0.00/s, evicted without access 0.00/s, random read ahead 0.00/s
lru len: 8136, unzip_lru len: 0
i/o sum[0]:cur[0], unzip sum[0]:cur[0]
--------------
row operations
--------------
0 queries inside innodb, 0 queries in queue
0 read views open inside innodb
main thread process no. 2345, id 140508950664960, state: sleeping
number of rows inserted 3919015, updated 0, deleted 0, read 8532141
0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
----------------------------
end of innodb monitor output
============================
 
1 row in set (0.00 sec)
 
mysql>

3.purge thread

事务提交后，purge thread线程回收已经分配并被使用的undo页。

innodb 1.2开始，支持多个purge thread。

purge thread离散地读取undo页。

mysql> show variables like 'innodb_purge_threads';
+----------------------+-------+
| variable_name        | value |
+----------------------+-------+
| innodb_purge_threads | 1     |
+----------------------+-------+
1 row in set (0.00 sec)
 
mysql>

4.page cleaner thread

1.2.x版本中引入的。将之前版本中脏页的刷新操作都放入到单独的线程中来完成。减轻master thread的工作。

总结

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